With the recent increase in the volume of information, there are ever-increasing commercial demands for increased speed and savings in material and manpower in the processing of such information. Information recording/reading technology is not an exception. A variety of recording/reading systems and improvements thereto have been put into practice.
The recording system employed in the information recording/reading technology will first be described hereinafter. The heat-sensitive recording system making use of heat or hot wires has found particularly wide-spread commercial utility. The heat-sensitive recording system includes (1) the hot pen system employed in the recording devices of medical analyzers; (2) the thermal head system incorporated in facsimiles, computer terminal equipment, and printers of desk-top electronic calculaters; and (3) the reflex baking system adopted in office copying machines. In each of the above systems, a heating element is brought into contact with heat-sensitive recording layers. As a result, there is unavoidable limitation to the resolution and recording speed due to the limited heat capacity of each heating element and the way of heat conduction.
With a view toward overcoming such drawbacks, a variety of heat mode recording systems have been proposed. These new recording systems make use of laser beams. Different from the conventional heat-sensitive recording systems, the laser beam recording system carries out recording without need for any direct contact between heating element and heat-sensitive recording layers. Hence, the laser beam recording system is free from the limitations stemming from the limited heat capacity of the heating element, the way of heat conduction, the limited size of the heating element in view of its machining, thereby permitting high-speed and high-density recording.
Roughly speaking, the laser heat mode recording system may be put to practice in two different ways. According to the first recording system, a difference in optical density is produced between a laser-irradiated spot and a laser-unirradiated spot by irradiating a laser beam onto a thin film-like recording means carried on a substrate such as film or the like to thermally excite the recording means at the irradiated spot, to induce fusion, evaporation or sublimation of the recording means, and to make the color of the spot extremely light or colorless. As thin film-like recording means useful in the practice of the first system, it has been proposed to use metals, organic polymers and coloring substances as materials to be fused, evaporated or sublimated. Among such recording means, metallic recording means are (1) expensive because a metal such as bismuth or indium has to be formed into a thin film on a substrate by the sputtering or vacuum evaporation method; and (2) accompanied by a low utilization efficiency of laser beam energy since many of such metallic recording means have a high reflectivity to laser beams. Correspondingly, metallic recording means require laser beams of a large output, whereby making the apparatus larger. They are thus not advantageous from the viewpoint of economy. On the other hand, a recording means using an organic polymer or coloring substance permits carrying out recording with laser beams of low energy since it has a low degree of thermal conductivity and a low sublimation temperature. However, a recording means making use of a polymer only has not yet succeeded in lowering to a satisfactory level the energy of laser beams required to perform the recording. Under the circumstances, it is still required to use a coloring substance having a strong capacity to absorb laser beams and converting them into heat, in combination with a polymer. Coloring substances of the above sort are required to have such a nature that they absorb heat and undergo fusion, evaporation or sublimation. As coloring substances having such a nature, there have heretofore been found blue and bluish as well as green and greenish coloring substances such as "Methylene Blue" and "Brilliant Green" for helium-neon laser beams, and red and reddish coloring substances such as "Ethyl Red" for argon laser beams. However, these coloring substances need gas laser beams, leading to large apparatus. Consequently, use of such coloring substances is not fully economical.
Accompanied with the rapid advance of semiconductor laser in recent years, it has become feasible to employ such semiconductor lasers as light sources for the recording systems of the aforementioned kind. However, no coloring substance has yet been found to be suitable for semiconductor laser beams. Carbon black and the like have been proposed, but they cannot provide high sensitivity due to their inherent particle shapes. Thus, there is a strong demand for the development of coloring substances suitable for semiconductor laser beams.
On the other hand, as the second laser heat mode system, it is possible to produce a difference in optical density between an irradiated spot and an unirradiated spot, as obtained in accordance with the above-mentioned first laser heat mode system, by irradiating a laser beam onto a thin film-like recording means, which has been obtained by further incorporating a compound capable or readily absorbing the laser beam and generating heat in a conventionally-known heat-sensitive recording means containing an organic polymer as a binder, a colorless or light-colored color-producing substance and a phenolic substance, and generating heat at the irradiated spot so as to produce a color there. As compounds capable of readily absorbing laser beams and generating heat which have heretofore been proposed for the second laser heat mode system, there are thionaphthol nickel complexes (see Japanese Patent Laid-open No. 121,140/1979). In order to draw out the effects of the above complexes in their entirety, it is however indispensable to employ laser beams having a wavelength at the long wavelength side of at least 1,100 nm. Thus, it is impossible to use practically-available semiconductor laser beams for them. Here again, there is a strong outstanding demand for the development of compounds which readily absorb laser beams and in turn generate heat.
Optical character recognition systems (OCR) have found their utility, particularly, as input units for computers and facsimiles. A variety of OCRs have been proposed so far, but their scanning (resolving) power of characters are not high enough. Use of laser beams as a light source of such an OCR has succeeded in greatly improving its resolving power because it makes effective use of the high focusability of laser beams and can use laser beams as a small point-source light of a spot size. When using a semiconductor laser as the light source of a scanner, it is however difficult to obtain strong contrast between light reflected by an image area such as character and an unrecorded area because images such as characters recorded with conventional ink do not absorb laser beams or, even if laser beams are absorbed, the extent of absorption is limited to a low level. It is possible to read out only characters or images recorded with a recording means containing carbon or "Nigrosine". However, neither carbon nor "Nigrosine" can be added to any conventional ink formulation. Therefore, it is difficult to read out characters or images which were recorded with an ordinary ball point pen or ink jet nozzle. Accordingly, development of reading means suitable for optical character recognition systems, which make use of a semiconductor laser as a light source for their scanner, have been awaited similar to the long standing demand for the development of recording means for the laser heat mode recording system.
With a view toward developing laser beam recording/reading means suitable to meet the aforementioned various demands, the present inventors carried out research with respect to various compounds which exhibit strong absorption of light in the near-infrared region. As a result, it was found that inorganic compounds such as phosphomolybdic acid and silicomolybdic acid are insoluble in solvents and thus unsuitable for the production of recording/reading means, and organic coloring substances such as cyanine family coloring substances are, when formed into thin films as recording/reading means, accompanied by such a drawback that their ability of absorbing light in the near-infrared region are weakened and eventually lost with the passage of time. Accordingly, the inventors' research was extended to organometallic complexes. As a result, it has been surprisingly found that benzenedithiol nickel complexes have a strong absorption band in the region of 800-950 nm and are not only soluble in organic solvents but also, when formed into thin films using an organic polymer as a binder, stable in their light absorbing ability with the passage of time. Furthermore, it has also been discovered that, upon irradiating a semiconductor laser beam of a wavelength in the region of 800-950 nm onto thin films, the benzenedithiol nickel complexes absorb the energy of the semiconductor laser beam and develop heat and, when the output of the laser beam is increased, the benzenedithiol nickel complexes are caused to evaporate together with the organic polymer. On the basis of the above findings, the present invention has been brought to completion.
An object of this invention is thus to provide a composition suitable for use in the production of such a laser heat mode recording medium and reading medium for the optical character recognition systems as mentioned above.